About 100,000 years ago, the barn swallow — that hairpin-turning, insect-snagging, eave-nesting icon of rural regions — took wing from northern Africa to begin blanketing the Northern Hemisphere.

Understanding how the barn swallow’s geographic range tuned the musical range of its song would likewise entail a sweeping quest: flying to multiple continents, recording 345 hours of the age-old classic, getting questioned by a Russian general.

Matt Wilkins was up for the challenge.

As with most of the 10,000-plus bird species, researchers know little about how geographic isolation — and the genetic differences evolving from that isolation — can shape the performance or composition of a barn swallow’s song.

Over the span of five years, Wilkins navigated the United States, Turkey, Israel, Romania, Taiwan and Russia to map those connections for the first time.

“For a bird with hundreds of studies on its mating habits and other aspects of its ecology, there was almost no research of its song,” said Wilkins, who wrote much of the resulting study as a postdoctoral researcher at the University of Nebraska-Lincoln. “But to get at the song aspect requires huge amounts of sampling, because the farthest distance between two populations is (about 7,000 miles).”

After a 73-day summer expedition across Russia coordinated by colleague Liz Scordato, the team found evidence that differences in the barn swallow song increase with geographic and especially genetic distance.

“Slight differences that accumulate at the genetic level when populations are isolated from each other (may have) caused more differences in the song, or the song is just diverging slowly in the same kind of clockwork fashion,” Wilkins said.

Most prior research on birds has failed to show that connection, he said. But many of those studies looked for genetic differences only at specific sites in a species’ genome, whereas Wilkins and his colleagues used more comprehensive sequencing techniques to examine “huge swaths of the DNA” – a scale to rival the team’s 4,000-mile trek from Moscow to Vladivostok.

Wilkins also discovered that the Siberian song features a couple of quirks that distinguish it from those of other subspecies, which tend to be very similar. The Siberian version ends with an additional, highly distinctive note. And the trill it shares with other subspecies is substantially shorter and slower, a surprising combination that Wilkins called “the worst of both worlds” when it comes to serenading a potential mate.

Why the differences? Around 27,000 years ago, the Siberian subspecies forked off from one that resides in North America, likely working its way to Russia by way of the Bering Strait. That bottleneck would have limited the population of barn swallows that recolonized Siberia, Wilkins said, amplifying the random genetic variation that could have produced a Siberian remix.

Six subspecies of barn swallow have evolved over time. As neighbors, the Eurasian and Siberian subspecies gave Wilkins the chance to study whether their songs change in areas where the two overlap – near Russia’s Lake Baikal, which holds one-fifth of the world’s freshwater. In some such cases, birdsongs have been shown to diverge even further, probably to help the subspecies avoid mistakenly breeding with each other. In other instances, those songs become more similar – a sort of lingua franca that allows the subspecies to better communicate and possibly negotiate territorial disputes. Wilkins and his colleagues found neither.

“That was kind of interesting, because it suggests that they’re already different enough that they don’t need to adapt their songs,” said Wilkins, now a postdoc at Vanderbilt University. “Or it could be that that they’re shifting their perceptions of the songs, which has been shown in singing mice – a cool thing that exists. In areas of overlap where (singing mice) compete with each other, the different subspecies can discriminate each other better in the area of overlap than they can where they’re separated.

“We’re always looking for the broad rules in biology, but I think we’re still pretty far from this one in terms of being able to understand the scenarios under which those different things happen.”

‘It didn’t occur to me that they were military helicopters…’

The countless hours that Wilkins and fellow researchers spent tracking barn swallows through rural Russia were nearly for naught, thanks to an unwelcome surprise at the very end of their journey.

“We’d sampled over 500 birds, and some of them were very difficult samples to acquire,” he said. “I’d been recording songs all this time. We’re almost to Vladivostok, with the blood samples and all of our measurements and everything, and we’re like, ‘We need one more data point.’”

So Wilkins, Scordato and Russian native Georgy Semenov began driving the countryside in search of structures that might house a barn swallow. Their reward: a pig farm.

“We were excited just to find anything,” Wilkins said. “I’ve got my super-sketchy spy equipment, with my binoculars and … this big parabola microphone designed for listening to people’s conversations across long distances. I’m pointing up at this wire with the microphone and talking to myself and looking through the binoculars.

“I keep seeing these helicopters, thinking, ‘These dang things keep screwing up my recording.’ It didn’t occur to me that they were military helicopters until later – Sikorsky-style with two big rotors. A little later, Georgy says, ‘Matt, I think you want to put that bird away. I think we’re going to have some company here.’”

Two vehicles sped toward the team. From the first emerged a man in camouflage; from the second, men adorned with the markings of a general and his second-in-command. The team had inadvertently stopped near a military training facility. Semenov began conversing with the general in Russian while Wilkins and Scordato remained silent, doing their best to look innocent.

“We’re just trying to nod at the appropriate times,” Wilkins said. “But I think I nodded at the wrong time, because the general does a double-take and and then says something to Georgy, who goes, ‘Get your passports.’ And we’re thinking, ‘Well, we’re not getting out of here.’

“(The general) was just baffled that we would be coming there to study birds. We convinced him, I guess, because our samples weren’t taken. But it was a really scary moment.”

Wilkins encountered the same bafflement elsewhere, including the Taiwanese capital of Taipei, where barn swallows have overcome their typical aversion to dense city environments and might be found perching next to a 7-Eleven. The city’s light pollution and near-constant traffic noise forced Wilkins to seek out specimens in the middle of the night, with the ticking clock of his 10-day visit a further reminder that the trip was more business than pleasure.

“It was no small challenge to even record those birds,” he said with a laugh. “You sort of burn the candle at both ends with that. You end up adopting the siesta. I was not seeing a lot of Taipei during the day.”

The team published its findings in the Biological Journal of the Linnean Society. Wilkins authored the study with the University of Colorado Boulder’s Rebecca Safran and Semenov; Scordato, now at California State Polytechnic University; Hakan Karaardiç from Alanya Alaaddin Keykubat University in Turkey; Daizaburo Shizuka, assistant professor of biological sciences at Nebraska; Alexander Rubtsov of the State Darwin Museum in Russia; Peter Pap of Babeş-Bolyai University in Romania; and Sheng-Feng Shen of Academia Sinica in Taiwan.

The researchers received support in part from the National Science Foundation.

Biological Sciences Ecology Evolution